Crystal Structure Analysis: Single-crystal X-ray diffraction studies are used to determine the crystal structures of calcium orthophosphates, calcium pyrophosphates, and other hydrated phosphates of biological interest. Crystal Chemistry of Calcium Phosphates: Emphasis is on the chemistry of octacalcium phosphate and how it participates in the formation of hard-tissue minerals and in the incorporation of impurities into such minerals. Studies are also carried out on animal models to establish whether octacalcium phosphate participates in the crystal growth mechanism in vivo. Cements and Remineralizing Agents: Studies are conducted on inorganic materials in aqueous systems that would be suitable as dental cements or as remineralizing agents for incipient dental caries lesions. Fluoride-Hydroxyapatite Interactions: Basic studies are carried out on the interaction of fluoride ions with hydroxyapatite crystal surfaces as part of a program to develop agents which will have improved properties as fluoridating agents. Calorimetric Studies: Heats of reaction of cations and of fluorides with the surface of hydroxyapatite crystals and of the hydrolysis of octacalcium phosphate are measured with a microcalorimeter.